High-performance, filler-reinforced, recyclable composite materials

ABSTRACT

Polyhexahydrotriazine (PHT) and polyhemiaminal (PHA) materials form highly cross-linked polymers which can be used as binder resins in composite materials. A filler element functionalized with a primary amine group can be covalently bonded to the PHA/PHT polymer resins. Example filler elements include, without limitation, carbon nanotubes, silica materials, carbon and glass fibers, and nanoparticles. Filler materials are incorporated into polymeric materials to improve the mechanical strength or other characteristics of the polymeric material for various applications. Typical composite materials use thermosetting materials that, once set, are intractable. PHT and PHA materials can be reverted to starting materials by exposure to acids. Thus, composite components formed using these materials are recyclable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/452,011, filed Aug. 5, 2014. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

INCORPORATION BY REFERENCE

Synthetic details and characterization of various examplepolyhexahydrotriazine and polyhemiaminal materials are provided incommonly assigned, co-pending application Ser. No. 14/050,995, filed inthe USPTO on Oct. 10, 2013, the entirety of which is incorporated hereinby reference.

BACKGROUND

The present disclosure relates to composite materials including fillercomponents.

For many applications, filler materials are incorporated into polymericmaterials to improve the mechanical strength or other characteristics ofthe polymeric material. For example, molded polymeric components may be“toughened” by incorporation of high-strength filler materials, such ascarbon nanotubes or carbon fibers. Similarly, composite materials, suchas glass-fiber reinforced plastic (“fiberglass”) comprising a polymericresin and a filler material, which may be a woven mat of fibers, havemany applications. Typically, these molded components and compositematerials use thermosetting polymeric resins, such as epoxy resins.However, the use of thermosetting resins make it generally difficultrework and/or recycle such molded components and composite materialsbecause once “set” these resins are highly crosslinked and, thus,generally intractable. Because the resins are intractable, the fillermaterials are typically not recoverable. In many instances, it isdesirable to recover filler materials for reuse since the fillers maybe, for example, expensive or otherwise difficult to procure.

In general, a need exists for high-performance composite materials andmolded components that are recyclable and/or re-workable.

SUMMARY

An exemplary polyhexahydrotriazine (PHT) resin comprises a plurality oftrivalent hexahydrotriazine groups having the formula (1):

anda plurality of divalent bridging groups of formula (2):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon, each starred bond of a givenhexahydrotriazine group is covalently linked to a respective one of thedivalent bridging groups, and each starred bond of a given bridginggroup is linked to a respective one of the hexahydrotriazine groups.

PHT resins can be prepared by a method comprising: forming a reactionmixture comprising a i) solvent, ii) paraformaldehyde, and iii) amonomer comprising two primary aromatic amine groups; and heating thereaction mixture at a temperature of 150° C. to about 280° C.

Another method for forming PHT resins comprises: forming a first mixturecomprising a i) solvent, ii) paraformaldehyde, and iii) a monomercomprising two primary aromatic amine groups; heating the first mixtureat a temperature of about 20° C. to about 120° C., thereby forming apolyhemiaminal (PHA) resin; and then heating the PHA resin at atemperature of 150° C. to about 280° C., thereby forming apolyhexahydrotriazine (PHT) resin.

An exemplary polyhemiaminal (PHA) resin comprises a plurality oftrivalent hemiaminal groups having the formula (3):

anda plurality of divalent bridging groups of formula (2):

wherein again L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon, each starred bond of a given hemiaminalgroup is covalently linked to a respective one of the divalent bridginggroups, and each starred bond of a given bridging group is linked to arespective one of the hemiaminal groups.

In an embodiment of the present disclosure, a composite materialcomprises a polyhexahydrotriazine (PHT) resin and a filler element thatis covalently bonded to the PHT resin. As used herein, “filler element”may be, without limitation, a tube, rod, sphere, bead, particle, orfiber. When a fiber (or otherwise similar in structure to a fiber), thefiller element may form a woven cloth, a mesh, a netting, a felt, or amat, or a portion of the foregoing. The filler element may comprise,without limitation, cellulosic material, silica, silicates, clay,carbonaceous materials (e.g., carbon black, graphite, and fullerenes),and metallic compounds. Individual filler elements may be of anydimension, for example, nanoscale and macroscale. A composite materialmay comprise a mixture of several different filler elements havingdifferent shapes, compositions, and/or sizes.

In another embodiment of the present disclosure, a method of forming acomposite material component includes forming a reaction mixtureincluding a monomer comprising two primary aromatic amine groups andparaformaldehyde, placing the reaction mixture in contact with a fillerelement having a primary amine group covalently bonded thereto.

In a further embodiment of the present disclosure, a method of recyclinga composite material component includes obtaining a composite materialcomponent including a PHT resin and a filler element that is covalentlybonded to the PHT resin, exposing the composite material component to anacid, and optionally recovering the filler element.

The above-described and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description, drawings, and appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts a reaction scheme for forming a composite materialincluding a filler element covalently bonded to a polyhexahydrotriazine(PHT) resin.

FIG. 2 depicts a filler element covalently bonded to apolyhexahydrotriazine (PHT) resin.

FIG. 3 depicts a method of forming a composite material component.

FIG. 4 depicts a scheme for recycling a composite material componentincluding a filler element covalently bonded to a polyhexahydrotriazine(PHT) resin.

FIG. 5 depicts a method for recycling a composite material componentincluding a filler element covalently bonded to a polyhexahydrotriazine(PHT) resin.

DETAILED DESCRIPTION

Methods are disclosed for preparing polyhemiaminal (PHA) resins andpolyhexahydrotriazine (PHT) resins by the reaction of aromatic amines,aromatic diamines, and paraformaldehyde. Aliphatic diamines may also bereacted in some embodiments.

PHA resins are stable intermediates in the preparation of the PHTresins. The PHA resins are generally prepared at a temperature of about20° C. to about 120° C., more preferably at about 20° C. to about 100°C., and further preferably at about 40° C. to about 60° C. PHA resinscan be cast from a polar aprotic solvents (e.g., N-methyl-2-pyrrolidone(NMP)), and PHA resins are stable at a temperature of about 20° C. toless than 150° C. The PHA resins can have a Young's modulus of about 6GPa, which is exceptionally high for an organic polymeric resin. PHAresins are also melt processable as well so can be used to in moldingand extrusion applications.

PHT resins can be formed by thermally treating a PHA resin at atemperature of at least 150° C., preferably about 165° C. to about 280°C., more preferably about 180° C. to about 210° C., and most preferablyabout 190° C. to about 210° C. for a period of time of about 1 minute toabout 24 hours, and more preferably about 1 hour. PHT resins can havehigh heat resistance as measured by dynamic mechanical analysis (DMA).PHT resins can also have a high Young's modulus as measured bynanoindentation methods. In some instances, the Young's modulus of a PHTmaterial can have a value in a range of about 8 GPa to about 14 GPa,exceeding that of bone (9 GPA).

PHT resins can also be formed in a more direct manner by the reaction ofaromatic diamines and paraformaldehyde at temperatures greater than usedin the formation of PHA resins, for example, at temperatures greaterthan 150° C. Under such conditions, the presumed PHA intermediary is notseparately isolated.

Herein, a polyhemiaminal (PHA) resin is a crosslinked polymer comprisingi) a plurality of trivalent hemiaminal groups of formula (3):

covalently linked to ii) a plurality of bridging groups of formula (4):

wherein y′ is 2 or 3, and K′ is a divalent or trivalent radicalcomprising at least one 6-carbon aromatic ring. Herein, starred bondsrepresent attachment points to other portions of the chemical structure.Each starred bond of a given hemiaminal group is covalently linked to arespective one of the bridging groups. Additionally, each starred bondof a given bridging group is covalently linked to a respective one ofthe hemiaminal groups.

As an example, a polyhemiaminal can be represented herein by formula(5):

In this instance, each K′ is a trivalent radical (y′=3) comprising atleast one 6-carbon aromatic ring. It should be understood that eachnitrogen having two starred wavy bonds in formula (5) is a portion of adifferent hemiaminal group.

In some embodiments, a polyhemiaminal (PHA) resin comprises, a pluralityof trivalent hemiaminal groups having the structure (3):

a plurality of bridging groups of formula (6):

anda plurality of monovalent end groups of formula (7):

wherein W′ is selected from the group consisting of: —H, —NH(R′),—N(R²)(R³), —OH, —O(R⁴), —S(R⁵), —P(R⁶), —CF₃, and combinations thereof,wherein R¹ comprises at least 1 carbon, R² comprises at least 1 carbon,R³ comprises at least 1 carbon, R⁴ comprises at least 1 carbon, R⁵comprises at least 1 carbon, R⁶ comprises at least 1 carbon, R⁷comprises at least one carbon, and each of R¹-R⁷ may be independent orthe same. Each starred bond of a given hemiaminal group is covalentlylinked to a respective one of the bridging groups or a respective one ofthe monovalent end groups. Each starred bond of a given bridging groupis linked to one of the hemiaminal groups. And each starred bond of agiven monovalent end group is linked to one of the hemiaminal groups.

As an example, a polyhemiaminal resin covalently bonded to a fillerelement can be represented herein by formula (8):

In this instance, each K′ is a trivalent radical (y′=3) comprising atleast one 6-carbon aromatic ring and K″ is a linking group covalentlyattached to the filler element. It should be understood that eachnitrogen atom having two starred wavy bonds in formula (8) is a portionof a different hemiaminal group. The inclusion of K″ reduces the numberof potential crosslink connection points in the polyhemiaminal resinnetwork; however the resin network is covalently bonded to fillerelement via the linking group K″. Covalent incorporation of the fillerelement into the resin network is important for achieving improvedcomposite material properties such as increased mechanical strength orrigidity because any separation or gap between filler and resin (binder)provides a potential crack or failure initiation point in the material.K″ may include, for example, at least one carbon. In some embodiments,K″ may be a six carbon alkyl chain.

Non-limiting exemplary trivalent bridging groups include:

The bridging groups can be used singularly or in combination. Theremainder of the description discusses divalent bridging groups K′. Itshould be understood that the methods and principles below also apply totrivalent linking groups.

Polyhemiaminals composed of divalent bridging groups K′ and monovalentend groups K″ can be represented herein by formula (9):

wherein K′ is a divalent radical (y′=2) comprising at least one 6-carbonaromatic ring, and K″ is and K″ is a linking group covalently attachedto the filler element. Each nitrogen atom having two starred wavy bondsin formula (9) is a portion of a different hemiaminal group. Theinclusion of K″ reduces the number of potential crosslink connectionpoints in the polyhemiaminal resin network; however the resin network iscovalently bonded to filler element via the K″ linking group. Covalentincorporation of the filler element into the resin network is importantfor achieving improved composite material properties such as increasedmechanical strength or rigidity because any separation or gap betweenfiller and resin (binder) provides a potential crack or failureinitiation point in the material. K″ may include, for example, at leastone carbon. In some embodiments, K″ may be a six carbon alkyl chain.

Certain bridging groups K′ have the formula (10):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, *—P(R′″)—* andcombinations thereof, wherein R′, R″, and R′″ independently comprise atleast 1 carbon. In an embodiment, R′, R″, and R′″ are independentlyselected from the group consisting of methyl, ethyl, propyl, isopropyl,phenyl, and combinations thereof. Other L′ groups include methylene(*—CH₂—*), isopropylidenyl (*—C(Me)₂-*), and fluorenylidenyl:

Also, as described above, a bridging group can also be of the formula(11):

Polyhemiaminal resin networks composed of divalent bridging groups offormula (11) and filler element linking groups K″ can be representedherein by formula (12):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, *—P(R′″)—* andcombinations thereof, wherein R′, R″, and R′″ independently comprise atleast 1 carbon. K″ is a linking group covalently bonded to a fillerelement. Each nitrogen atom having two starred wavy bonds in formula(12) is a portion of a different hemiaminal group.

An embodiment of a polyhexahydrotriazine (PHT) composite resin,comprises, i) a plurality of trivalent hexahydrotriazine groups offormula (13):

covalently linked to ii) a plurality of divalent bridging groups offormula (14):

(where y′=2 or 3) (14); andiii) a plurality of linking groups K″ of formula (15):

K″—*  (15),

Each starred bond of a given hexahydrotriazine group is covalentlylinked to a respective one of the bridging groups K′ or a respective oneof the linking groups K″. K′ is a divalent or trivalent radicalcomprising at least one 6-carbon aromatic ring, and K″ is linking groupcovalently bonded to a filler element. In an embodiment, K″ includes atleast one carbon atom. In another embodiment, K″ is an alkyl chainincluding at least six carbon atoms. Each starred bond of a givenbridging group or a given linking group is covalently linked to arespective one of the hexahydrotriazine groups.

For PHTs comprising bridging groups of formula (14) and linking groupsK″, the polyhexahydrotriazine composite resin is represented herein byformula (16):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. K″ is again linking group covalently bonded to a fillerelement. Each nitrogen having two starred wavy bonds in formula (16) isa portion of a different hexahydrotriazine group.

Exemplary non-limiting divalent bridging groups include:

and combinations thereof.

The PHA and PHT resins can further comprise monovalent aromatic groups(referred to herein as diluent groups), which do not participate inchemical crosslinking and therefore can serve to control the crosslinkdensity as well adjust the physical and mechanical properties of the PHAand PHT resins. Monovalent diluent groups have a structure according toformula (17), formula (18), formula (19), and/or formula (20):

wherein W′ is selected from the group consisting of: —H, —NH(R¹),—N(R²)(R³), —OH, —O(R⁴), —S(R⁵), —P(R⁶), —R⁷, —CF₃, and combinationsthereof, wherein R¹ comprises at least 1 carbon, R² comprises at least 1carbon, R³ comprises at least 1 carbon, R⁴ comprises at least 1 carbon,R⁵ comprises at least 1 carbon, R⁶ comprises at least 1 carbon, R⁷comprises at least one carbon, and each of R¹-R⁷ may be independent orthe same. The starred bond is linked to a nitrogen of a hemiaminal groupor a nitrogen of a hexahydrotriazine group.

Non-limiting exemplary diluent groups include:

wherein the starred bond is linked to a nitrogen of a hemiaminal groupor a hexahydrotriazine group. Diluent groups can be used singularly orin combination.

The reactivity of a given diluents (end group precursor) may varyaccording to whether the substituent(s) attached to aromatic ring areelectron rich or electron poor. In general, more strongly electronwithdrawing (electron poor) substituents reduce reactivity of themonomer and more strongly electron donating (electron rich) substituents increase reactivity. As such, it is possible to control theratio of different end groups in the final product by selecting endgroup precursors on the basis of expected reactivity (and/or adjustingfeed ratios). Additionally, electron poor substituents react more slowlyand can be used to vary the character of the reaction end-productbetween hemiaminal and hydrotriazine. That is, less reactive (electronpoor) monomer units will tend produce a reaction product having morehemiaminal groups as compared to more reactive (electron rich) monomerunits.

The ratio of bridging groups and end groups in the final resin cansimilarly be adjusted using the relative reactivity of the bridginggroup monomers and the end group monomers. A more reactive end group(e.g., one with a substituent which is electron rich) will tend toreduce cross-link density and molecular weight in the final resin. Aless reactive end group (e.g., one with a substituent which is electronpoor) will tend to increase crosslink density because fewer hemiaminalor hexahydrotriazine groups will bond to such an end group and willinstead be bonded only to bridging groups, which results in crosslinks.

PHA Composite Materials

A method of preparing a polyhemiaminal (PHA) composite materialincluding divalent bridging groups comprises steps of: forming a firstmixture comprising: i) a first monomer comprising two or more primaryaromatic amine groups (e.g., corresponding to K′ with primary amines(—NH₂) at the * locations)), ii) (optionally) a second monomer havingonly one aromatic primary amine group (i.e., a diluent group), iii)paraformaldehyde, and iv) a solvent. The first mixture is thenpreferably heated at a temperature of about 20° C. to about 120° C. forabout 1 minute to about 24 hours, in the presence of primary aminesurface-functionalized filler element, thereby forming a second mixtureincluding a PHA composite material with a covalently bonded fillerelement therein.

The mole ratio of paraformaldehyde to total moles of primary aromaticamine groups (e.g., 2× moles diamine monomer+1× moles monoaminemonomer+1× primary amine reaction sites on the filler element) ispreferably about 1:1 to about 1.25:1, based on one mole ofparaformaldehyde equal to 30 grams.

Non-limiting exemplary first monomers comprising two primary aromaticamine groups include 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline(MDA), 4,4′-(9-fluorenylidene)dianiline (FDA), p-phenylenediamine (PD),1,5-diaminonaphthalene (15DAN), 1,4-diaminonaphthalene (14DAN), andbenzidene, which have the following structures:

Non-limiting exemplary second monomers having only one primary amineinclude N,N-dimethyl-p-phenylenediamine (DPD), p-methoxyaniline (MOA),p-(methylthio)aniline (MTA), N,N-dimethyl-1,5-diaminonaphthalene(15DMN), N,N-dimethyl-1,4-diaminonaphthalene (14DMN), andN,N-dimethylbenzidene (DMB), which have the following structures:

The second monomer is optional can be used in an amount of 0 mole % toabout 99 mole % based on total moles of first monomer and secondmonomer. In a particular embodiment, the second monomer can be used inan amount of 10 mole % to about 67 mole % based on total moles of firstmonomer and second monomer.

The solvent can be any suitable solvent. Preferred solvents includedipolar aprotic solvents such as, for example, N-methyl-2-pyrrolidone(NMP), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMA), propylene carbonate (PC), and propyleneglycol methyl ether acetate (PGMEA). In a typical embodiment, thesolvent is NMP.

PHT Composite Materials

A method of preparing a polyhexahydrotriazine (PHT) comprises forming afirst mixture comprising i) a first monomer comprising two aromaticprimary amine groups, ii) (optionally) a second monomer having only onearomatic primary amine group, iii) paraformaldehyde, and iv) a solvent,and heating the first mixture at a temperature of at least 150° C.,preferably about 165° C. to about 280° C., in the presence of a primaryamine surface-functionalized filler element, thereby forming a secondmixture comprising PHT composite material with a covalently bondedfiller element therein. The heating time at any of the abovetemperatures can be for about 1 minute to about 24 hours.

Alternatively, the PHT composite material can be prepared by heating aPHA composite material (prepared as described above) at a temperature ofat least 150° C., preferably about 165° C. to about 280° C. even morepreferably at about 180° C. to about 220° C., and most preferably atabout 200° C. for about 1 minute to about 24 hours.

FIG. 1 depicts a reaction scheme for forming a composite materialincluding a filler element covalently bonded to a polyhexahydrotriazine(PHT) resin. As depicted in FIG. 1, the filler element is a single-wall,carbon nanotube. This nanotube, at an initial stage, has primary aminegroups covalently linked thereto via a linking group “R.” In thisexample, “R” includes at least one carbon atom. More specifically, inthis example, “R” is a six carbon, straight alkyl chain. Reactivity ofthe primary amine groups may be somewhat reduced for shorter chainlinking groups. ODA is selected as the divalent bridging group. Reactionof ODA with paraformaldehyde in the presence of the functionalizednanotube under the conditions discussed above results in the formationof a PHT composite material. Though the filler element is depicted withtwo primary amine reaction sites, this is for purposes of explanation.The number of reaction sites per filler element is not limited to twosites and may be any suitable number greater than or equal 1. It wouldbe expected for the total number of sites to vary with such things asdesired end-use properties of the composite material, the size of thefiller element, the reactivity of the reaction sites, and the like.

FIG. 2 depicts a PHT composite material including a filler element 200covalently bonded into a polyhexahydrotriazine (PHT) resin. Fillerelement 200 may be, without limitation, a tube, rod, sphere, bead,particle, or fiber. When a fiber (or otherwise similar in structure to afiber), the filler element may form a woven cloth, a mesh, a netting, afelt, or a mat, or a portion of the foregoing. The filler element maycomprise, without limitation, cellulosic material, silica, silicates,clay, carbonaceous materials (e.g., carbon black, graphite, andfullerenes), and metallic compounds. In some embodiments, the fillerelement may comprise carbon-containing materials such as carbonnanotubes, carbon fibers, fullerenes, and woven mats. In otherembodiments, the filler element may comprise silica-containing fillermaterials, including silica spheres and silica fibers. In additionalembodiments, the filler element may comprise inorganic filler materials,such as silica, alumina, titanium dioxide. Individual filler elementsmay be of any dimension, for example, nanoscale and macroscale. Acomposite material may comprise a mixture of several different fillerelements having different shapes, compositions, and/or sizes.Amine-modified fillers are available commercially and/or may be producedaccording to methods known in the art.

As depicted in FIG. 2, the PHT resin is a material including a divalentbridging group wherein L′ is a divalent linking group selected from thegroup consisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. However, in general, the PHT resin (binder) in a PHT compositematerial may be any of PHTs disclosed or suggested herein. Similarly, inother embodiments, the PHT resin (binder) in FIG. 2 may be replaced witha PHA material disclosed or suggested herein.

In some embodiments, the filler element may be covalently linked to thebinder resin by placing the filler element in contact with a reactionmixture, which may include a solvent, a diamine monomer, andparaformaldehyde. In other embodiments, the filler element may becovalently linked to the binder resin by melt mixing PHA/PHT resins andthe filler element at elevated temperatures without use or presence of asolvent.

Applications of Composite Materials

The PHA and PHT composite materials are attractive for applicationsrequiring lightweight, rigid, strong, heat resistant components such asaerospace engineering, automotive components, electronics, or the like.In general, PHT materials have unique combinations of properties thatinclude high modulus, solvent resistance, heat resistance/insensitivity,resistance to stress fracture/cracking. Additionally, unlike mostthermosetting materials, PHA/PHT materials can be reverted to monomervia treatment with an acid. This allows PHA/PHT composite materials (orcomponents formed by these materials) to be reworked or recycled tostarting materials.

FIG. 3 depicts a method of forming a composite material componentaccording to an embodiment.

In element 310, a primary amine functionalized filler element (e.g.,filler element 200) is obtained. The filler element may have beenpreviously functionalized or it may be functionalized at this time.

In element 320, a PHA/PHT precursor (“first monomer” such as ODA or PD)is mixed with paraformaldehyde (PF) and the functionalized fillerelement to form a reaction mixture. Optional diluents groups (“secondmonomer”) and a solvent may also be included in the reaction mixture.

In element 330, heating and/or molding of the reaction mixture is usedto form a composite component. Heating of the reaction mixture causesthe composite material to set—that is, a crosslinked resin material isformed and the filler element is covalently bonded to this resinmaterial. Element 330 may occur in stages. For example, the reactionmixture may be heated to a first temperature to cause a PHA compositematerial to form, then this reaction product may be processed (cast,molded, injected, formed, etc.) to form the composite component into thedesired shape or structure. After such processing (or during suchprocessing) the PHA composite material may be heated to a secondtemperature at which the PHA composite material converts to a PHTcomposite material. Alternatively, the heating and molding may occur ina single stage and a composite component formed of a PHT compositematerial may be fabricated directly.

In optional element 340, the composite component may be exposed to anacid and reworked. In this context, “reworked” means that if thiscomposite component is misshaped or positioned in an incorrect location,the composite component can be reshaped or removed from incorrectlocations by exposure to an acid, rather than scrapping the entirecomposite component. In general, when a thermosetting resin, such as anepoxy resin is used in a composite component, it is not possible torework the composite component after the epoxy resin sets. However, PHAand PHT materials can revert to monomer components upon exposure toacids (e.g., H₂SO₄, acetic acid).

FIG. 4 depicts a scheme for recycling a composite material componentincluding a filler element covalently bonded to a polyhexahydrotriazine(PHT) resin. The PHT composite material formed according to the schemedepicted in FIG. 2, reverts to starting materials (i.e., “first monomer”and carbon nanotube) upon exposure to an acid. The breakdown of thecomposite material by this method allows for reworking, recycling, anddisposal of composite components formed using PHT binder resins.Traditional means for recycling or disposing of thermoset compositecomponents uses mechanical grinding or thermal degradation. Thesemethods do not return the composite component to reusable startingmaterials. In many applications, it would be desirable to recover andreuse filler elements, which may be high value materials, such as carbonnanotubes (or other fullerene molecules). The presently disclosedmaterials and methods allow for the recovery of high value fillerelements.

FIG. 5 depicts a method for recycling a composite material componentincluding a filler element covalently bonded to a polyhexahydrotriazine(PHT) resin.

In element 510, a composite material component including a fillerelement covalently bonded to a PHT resin is obtained. In thisembodiment, the composite component is formed by a process comprisingelements 512, 514, and 516. In this process, a filler element's surfaceis functionalized with a primary amine group (512), the functionalizedfiller and PHA/PHT precursor materials (e.g., “first monomer” and“second monomer” disclosed above) are mixed and reacted to form acomposite material (514), the composite material is heat and/or moldedto form a composite component (516). In other embodiments, element 510may simply be obtaining an already formed composite component. Forexample, the composite component may be a discarded or obsoletecomponent from an automobile being scrapped. In other embodiments,element 514 and element 516 can occur simultaneously such that thecomposite material and the molded component are formed in a singleprocessing step. Element 514 can also comprise a melt mixing process inwhich the binder resin (or the binder resin in a pre-polymer or lowmolecular weight initial state) is mixed with the functionalized fillerto provide the composite material.

In element 520, the composite component is exposed to an acid andconsequently reverted to starting materials (i.e., the PHA/PHTprecursors and the functionalized filler), such as, depicted in FIG. 4.

In element 530, the filler element may be optionally recovered for reuseor special disposal processing. The recovery process may involve, forexample, a centrifuge process, solvent extraction, evaporativedistillation, or other separation techniques. When the filler materialis nanoparticles or the like, special disposal processing may beimplemented so as to avoid release of nanoparticle filler elements intothe environment.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. When a range is used to express apossible value using two numerical limits X and Y (e.g., a concentrationof X ppm to Y ppm), unless otherwise stated the value can be X, Y, orany number between X and Y.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and their practical application, and toenable others of ordinary skill in the art to understand the invention.

What is claimed is:
 1. A composite material, comprising: apolyhemiaminal (PHA) resin; and a filler element covalently bonded tothe PHA resin.
 2. The composite material of claim 1, wherein the PHAresin, comprises: a plurality of trivalent hemiaminal groups having thestructure

and a plurality of divalent bridging groups of formula:

wherein L′ is a divalent linking group selected from the groupconsisting of —O—, —S—, —N(R′)—, —N(H)—, —R″—, and combinations thereof,wherein R′ comprises at least 1 carbon and R″ comprises at least onecarbon, each starred bond of a given hemiaminal group is covalentlylinked to a respective one of the divalent bridging groups or the fillerelement, and each starred bond of a given bridging group is linked to arespective one of the hemiaminal groups.
 3. The composite material ofclaim 2, wherein L′ is one of —O— and —S—.
 4. The composite material ofclaim 2, wherein L′ is —N(R′)—, wherein R′ is selected from the groupconsisting of methyl, ethyl, propyl, isopropyl, phenyl, and combinationsthereof.
 5. The composite material of claim 2, wherein L′ is —CH₂—. 6.The composite material of claim 2, wherein L′ is 9-fluorenylidenyl:


7. The composite material of claim 1, wherein the PHA resin includes atrivalent bridging group.
 8. The composite material of claim 1, whereinthe PHA resin includes least one diluent group is selected from thegroup consisting of

and combinations thereof, wherein W′ is a monovalent radical selectedfrom the group consisting of —H, —NH(R′), —N(R²)(R³), —OH, —O(R⁴),—S(R⁵), —P(R⁶), —R⁷, —CF₃, and combinations thereof, wherein R¹, R², R³,R⁴, R⁵, R⁶, and R⁷ each comprise at least 1 carbon and each of R¹-R⁷ maybe independent or the same, and the starred bond in each of the diluentsgroups linked to a nitrogen of a hemiaminal group of the PHA resin. 9.The composite material of claim 1, wherein the filler element is afullerene.
 10. The composite material of claim 9, wherein the fullereneis a single-walled nanotube.
 11. The composite material of claim 1,wherein the filler element is a carbon fiber.
 12. The composite materialof claim 1, wherein the filler element is a glass fiber.
 13. A method ofrecycling a composite component, comprising: obtaining a compositecomponent including: a polyhemiaminal (PHA) resin, and a filler elementcovalently bonded to the PHA resin; and exposing the composite componentto an acid.
 14. The method of claim 13, further comprising: recoveringthe filler element.
 15. The method of claim 13, wherein the fillerelement is a nanoparticle.
 16. The method of claim 13, wherein the PHAresin comprises: a plurality of trivalent hemiaminal groups having thestructure

and a plurality of divalent bridging groups of formula:

wherein L′ is a divalent linking group selected from the groupconsisting of —O—, —S—, —N(R′)—, —N(H)—, —R″—, and combinations thereof,wherein R′ comprises at least 1 carbon and R″ comprises at least onecarbon, each starred bond of a given hemiaminal group is covalentlylinked to a respective one of the divalent bridging groups or the fillerelement, and each starred bond of a given bridging group is linked to arespective one of the hemiaminal groups.
 17. A method of recycling acomposite component, comprising: obtaining a composite componentincluding: a polyhemiaminal (PHA) resin, and a carbon nanotubecovalently bonded to the PHA resin; and exposing the composite componentto an acid.
 18. The method of claim 17, further comprising: recoveringthe carbon nanotube.
 19. The method of claim 17, wherein the carbonnanotube is a single-walled nanotube.
 20. The method of claim 17,wherein the PHA resin comprises: a plurality of trivalent hemiaminalgroups having the structure

and a plurality of divalent bridging groups of formula:

wherein L′ is a divalent linking group selected from the groupconsisting of —O—, —S—, —N(R′)—, —N(H)—, —R″—, and combinations thereof,wherein R′ comprises at least 1 carbon and R″ comprises at least onecarbon, each starred bond of a given hexahydrotriazine group iscovalently linked to a respective one of the divalent bridging groups orthe filler element, and each starred bond of a given bridging group islinked to a respective one of the hexahydrotriazine groups.